Addiction to opioid drugs has become a substantial public health problem due to the large number of Americans who use opioids for chronic pain, and the increased use by teenagers and adults of opioid prescription painkillers for nonmedical purposes. In this application, we describe development of a behavioral paradigm that models 'addiction-like' behavior for opioid drugs, we link this behavior with the expression of key molecular components, and we outline a novel strategy to translate this knowledge into potential therapeutic interventions. Specifically, using this behavioral paradigm, we linked 'addiction-like' behavior for heroin in rats following a one month test period to expression of several key regulators of the mu-opioid receptor (MOR), the primary mediator of the analgesic and rewarding properties of opioid drugs. One of these regulatory proteins is Wntless (WLS), a transport protein necessary for Wnt protein secretion. Wnts are ubiquitous glycoproteins with trophic properties for neurons, including dendritic spine maintenance and increased hippocampal neurogenesis, properties that are normally inhibited by opioid agonist drugs such as morphine, heroin, and fentanyl. Binding of morphine to the MOR enhances the interaction between MOR and WLS, resulting in the inhibition of Wnt protein secretion. Changes in WLS expression were also found to occur in the prefrontal cortex of rats demonstrating addiction-like behaviors for heroin. These results lead to the hypothesis that the opioid-induced MOR/WLS interaction is a critical molecular component contributing to the development of opioid addiction. Experiments proposed in this application are designed to challenge the MOR/WLS hypothesis by testing whether blockade of the MOR/WLS interaction in the prefrontal cortex can prevent acquisition of heroin 'addiction-like' behaviors in rats and/or rescue heroin-experienced rats from 'relapse'. Specific Aim 1 will test whether the disruptive effects of morphine on Wnt secretion can be prevented in vitro via administration of MOR/WLS blocking peptides expressed in a lentiviral vector. Specific Aim 2 will use a viral vector to test whether blockade of the MOR/WLS interaction in the prefrontal cortex will disrupt the development of addiction-like behavior for heroin in drug nave rats, while Specific Aim 3 will use a viral vector to test whether blockade of the MOR/WLS interaction in the prefrontal cortex will disrupt drug-induced reinstatement of heroin-seeking behavior in heroin-experienced rats. If successful, results from the experiments proposed in this application will reveal a novel avenue for the pharmacotherapy of opioid addiction in humans.